Unprecedented Public Safety Plan for Obama Inauguration

Virtually every major bridge heading into the nation's capital on Inauguration Day will be closed to vehicular traffic. In other words, unless they are on a bus or in an authorized vehicle, riders will be turned away by the police or the military.

That announcement came from the United States Secret Service, which is the organization tasked with handling security for the inauguration of Barack Obama as the 44th president of the United States.

The reasons for all the controls over traffic became clear Wednesday night as ABC News learned of a new security bulletin regarding the inauguration.

While there is no specific credible threat to the event, the government warning expressed concern about improvised bombs, suicide attacks and hostage scenarios. The reason? Intelligence officials have concluded that the inauguration is a very "attractive target" -- because of the symbolism, its historic nature, the presence of VIPs and the sheer volume of people expected in and around the ceremonies.

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"There are no adjustments being made at this time to the nation's threat level," read the statement on the joint threat assessment by the Department of Homeland Security, the FBI, the National Counterterrorism Center, the National Geospatial Intelligence Agency, the U.S. Northern Command and fusion centers in the National Capital Region.

"As a routine matter, we also remind the public planning to attend the inauguration to be both thoughtful and vigilant of their surroundings, and to report anything suspicious to authorities," the statement added.

The most probable targets are thought to include crowds on the mall, hotels and restaurants and streets heading into the parade area.

All people along the parade route and at the balls will have to go through security screening. Some of the items restricted along the parade route beyond the obvious prohibition against weapons include aerosols, supports for signs and placards, packages, coolers, thermal or glass containers, backpacks, bags exceeding size restrictions (8" x 6" x 4"), laser pointers, animals other than helper/guide dogs, structures, bicycles and any other items determined to be a potential safety hazard.

In addition to vehicular restrictions, the U.S. Coast Guard will have enhanced patrols of the waterways and the military will patrol restricted airspace.

There will be a display of uniforms and also undercover cops posing as tourists on the mall.

Eight thousand police -- 4,000 from the D.C. Metro Police Department and 4,000 from 96 other departments around the country -- will join an unspecified number of federal agents from the FBI, Secret Service and other agencies.

Also on hand will be 11,000 military personnel, including fighter jet squadrons and units specializing in responding to biological, radiological and chemical attacks.

All kinds of sensors will be deployed looking for any hint of an attack. Satellites will also be used. Washington police have also been adding more security surveillance cameras.

Officials are pulling out all the stops in part because of the inauguration's historic nature and the expected unprecedented crowd. Of great concern is the sheer -- likely unheard of -- concentration of people. Estimates vary, but some officials believe there will be more than 2 million people packed on the National Mall.

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